1. Field of the Invention
The present invention relates to a navigation device and a control method for displaying a detour and, in particular, to a navigation device and a control method for displaying a detour when a traffic hazard, for example, a traffic jam occurs on a guidance route.
2. Description of the Related Art
A car-mounted navigation device generally detects a current location of the vehicle using an autonomous navigation sensor, a Global Positioning System (GPS) receiver, or the like and reads map data near the location from a storage medium to display a map on a screen. Then, a driver's location mark that shows the vehicle location is also superimposed and displayed at a predetermined point on the screen such that the location where the vehicle currently runs can be found at a glance.
Further, the navigation device includes a route guidance function. The route guidance function automatically explores a lowest-cost route connecting the current location to the destination using the map data and draws the explored route as a guidance route on the map screen with a different color from the other routes and with a thick line. Further, when the vehicle approaches a point within a predetermined distance from an intersection to which the vehicle is guided on the guidance route, the device displays an enlarged map of the intersection to show the information about the intersection in such a way as to guide the driver to a destination.
Further, most of navigation devices include a traffic information providing function. The navigation device can receive the road traffic information sent from a road traffic information center and display the changing road condition on the screen using the traffic information providing function. A navigation device including such a traffic information providing function can provide a user with the road traffic information including the information about a traffic jam or the information about a regulation in real time.
Some of the navigation devices including such route guidance function and traffic information providing function also include a function for automatically exploring a detour to avoid a traffic jam according to the received road traffic information (see JP 09-042984 A and JP 2006-275729 A, for example). The technique disclosed in JP 09-042984 A is configured to explore an available detour from the intersection to which the vehicle is guided on the guidance route and display the detour when the vehicle approaches the intersection to which the vehicle is guided. The technique disclosed in JP 2006-275729 A is configured to store information about the frequently-used shortcuts and back roads in the map data to selectively extract only the shortcut or the back road to go along the guidance route toward the destination from among the shortcuts and back roads and display the extracted shortcut or back road on the screen.
Map data used for a navigation device is generally layered in units referred to as levels and managed. The levels include a higher level at which a map shows a panoramic view of a wide area, and a lower level at which a map depicts the detail of a narrow area. The map data at one of the layers is read according to the scale designated by the user and then a map is displayed on the screen based on the map data. The smaller scale the wide area map is on, the fewer the elements (roads, icons or the like) are displayed on the map. The larger scale the detailed map is on, the more the elements are displayed on the map.
Accordingly, there is a problem in that a road that is not included in the map data at the level corresponding to the currently designated scale (currently displayed on the screen) cannot be displayed as a detour even if a detour is explored near the intersection to which the vehicle is guided as described in JP 09-042984 A. Similarly, there is a problem in that a road that is not included in the map data at the level corresponding to the currently designated scale cannot be displayed as a shortcut or a back road even if a shortcut or a back road is displayed as described in JP 2006-275729 A.
There is a problem in that, if it is desirable that narrow roads such as a narrow street are displayed as detours, shortcuts or back roads, it is necessary to change the designation of the scale to a level at which the detailed map of the city is displayed, and this complicates the operation. Further, there is a problem in that, if the scale is increased to a level at which the detailed map of the city is displayed, it is difficult to see an overview the detour, the shortcut, or the back road. This brings a feeling of anxiety to the driver because it is difficult for the driver to know the route of the detour or how far is the detour. A road having a width of less than 5.5 m is referred to as a narrow street in Japan. The narrow street corresponds to a road referred to as a minor street or a secondary street in the United States.